利用共振腔振盪衰減光譜法研究CH3OO在C-H伸展振動模光區的紅外吸收光譜

Abstract

利用波長為193nm的雷射光解流動的CH3C(O)CH3/O2混合氣體和波長為248nm的雷射光解流動的CH3I/O2混合氣體，並以共振腔振盪衰減法取得其共同產物CH3OO的紅外吸收光譜。吾人將2953.4cm-1和3020.7cm-1的吸收峰分別指派為CH3OO的ν2和ν9，與黃登瑞等人得到同為氣態環境下的CH3OO低解析度光譜之ν2和ν9的位置2954cm-1和3020cm-1一致。此結果亦和Nandi與Morrison兩研究組分別在Ar間質與He奈米液滴環境下得到ν2和ν9的相對位置一致，並與B3LYP/aug-cc-pVTZ計算得到之非簡諧振動頻率相差在1%以內。在吾人光譜中並未能指認出可能是ν1的吸收峰，可能是ν1譜線較弱且結構缺乏一明顯的吸收峰所致。此外，吾人以近似對稱陀螺分子之模式，分析CH3OO的轉動譜線結構而得振動基態及ν2和ν9的轉動常數，和Endo研究組以及理論計算得到之結果一致。吾人利用SpecView光譜模擬程式模擬ν2和ν9之光譜並與實驗光譜做對照，亦模擬ν1並討論其可能的躍遷原點位置。對ν2而言，模擬光譜大致上和實驗光譜吻合，但因模擬軟體未考慮ν2可能受到coriolis coupling之影響，導致模擬光譜和實驗光譜在2940-2950cm-1的光區範圍內有所差異。而ν9的實驗光譜在3014cm-1和3017cm-1出現沒有對應到模擬光譜之譜線，且在3025-3050cm-1之光區，實驗和模擬光譜並不一致，吾人推測這些不一致的譜線應是受到ν1吸收之影響。經由改變ν1模擬光譜的躍遷原點並將其和實驗光譜比較，吾人暫時將ν1的躍遷原點指派為3031.7cm-1。此外ν9的半高寬比SpecView模擬程式預測的半高寬為大，應是和ν1譜線重疊，或是CH3OO內轉動運動造成的譜線分裂所致。而此內轉動運動所引起的譜線分裂，亦是造成ν1的Q分枝和模擬光譜不一致的可能原因。ν2的平行躍遷扭動分裂極小，因此譜線半高寬和模擬光譜所得之結果一致。

We employed a cavity ringdown spectrometer with a tunable infrared OPO/OPA laser with a bandwidth of 0.02 cm-1 to record the absorption spectra of methylperoxy radicals (CH3OO) in the range 2930-3050 cm-1. Methylperoxy radicals were produced by irradiating a flowing mixture of CH3I and O2 with emission at 248 nm from a KrF excimer laser or a flowing mixture of CH3C(O)CH3 and O2 with emission at 193 nm from an ArF excimer laser. Two absorption bands with origins at 2953.4 cm-1 and 3020.7 cm-1 were observed; they are assigned to ν2 (symmetric CH3 stretching) and ν9 (antisymmetric CH2 stretching) modes of CH3OO, respectively. We analyzed the rotational structures of the ν2 and ν9 bands by simply treating CH3OO as a prolate symmetric top, and determined the rotational constants both in the ground state and in the vibrationally excited state. We predicted vibrational wavenumbers and rotational parameters for the upper and lower vibrational states, and the mixing ratio among a-, b-, c-types of bands of CH3OO with the B3LYP/aug-cc-pVTZ density-functional theory. The rotational contours for the ν1, ν2 and ν9 bands of CH3OO were simulated with the SpecView software. For the ν2 band, the simulation agrees satisfactorily with the experimental observations except for the intense peaks with regular spacing about 2.4 cm-1 in the range 2940-2950 cm-1. For the ν9 band, the simulation result is consistent with the experimental observations in the region 3000-3020 cm-1 but not in the region 3020-3050 cm-1. The discrepancy might be due to the interference from the ν1 band. That ν1 band is unobserved is likely due to its relatively small intensity. We temporarily assigned the ν1 band to be at 3031.7 cm-1 by matching the simulated spectra with the peaks which do not correspond to the ν9 band.